The mechanical environment of the chondrocytes is one of several important environmental factors which regulate the normal balance between the synthesis and degradation of the articular cartilage extracellular matrix. Chondrocytes are surrounded by a pericellular matrix, which together with the chondrocyte and a surrounding capsule have been termed the "chondron". The functional role of this complex structural unit of cartilage is not known. The central hypothesis of this study is that the pericellular matrix plays a significant biomechanical role in regulating the mechanical stress environment of the chondrocyte. This hypothesis will be tested by combining detailed measurements of the mechanical properties of the pericellular matrix of isolated chondrons, with a finite element model of the chondron within articular cartilage. In this manner, the applicants propose to show, theoretically, that the mechanical properties of the pericellular matrix significantly affect the stress-strain and fluid flow environments of the chondrocyte; they propose to show, experimentally, that the presence of the in vivo pericellular matrix alters the chondrocytes' metabolic response to mechanical stress. Further, it is proposed that the mechanical properties of the pericellular matrix are changed with osteoarthritis, resulting in an alteration in the mechanical environment of the chondrocyte. It is expected that these alterations in the mechanical environment will affect the chondrocytes' ability to regulate proteoglycan synthesis rates in response to mechanical stress. These experiments will be performed using an isolated chondron model from adult human cartilage. The following Specific Aims will be completed: 1) to combine novel micromechanical experiments (micropipette aspiration and atomic force microscopy), with theoretical modeling, to determine the biphasic mechanical properties of the pericellular matrix in isolated chondrons. These measured properties will be utilized in a finite element model of the chondron in the extracellular matrix to quantify the mechanical stress environment of the chondrocyte. The theoretical predictions of chondron deformation will be validated, in situ, using three-dimensional confocal microscopy; 2) to use these methods to quantify the changes that occur in the biphasic properties of the pericellular matrix with osteoarthritis; 3) to determine the effects of enzymatic degradation on the mechanical properties of the pericellular matrix; and 4) to quantify the role of the pericellular matrix in regulating the rate of proteoglycan metabolism in response to stress, using isolated chondrocytes and chondrons compressed in an alginate matrix. The long-term goal of these studies is to better understand the mechanisms through which mechanical factors influence cartilage maintenance in both health and disease. Identification of these mechanisms will hopefully lead to new treatments which exploit optimal mechanical and biochemical modalities for the prevention of osteoarthritis.